The present invention relates to an improvement in automobile air conditioner compressors of the rotary type which maintains the output of the compressor and thereby the cooling effect of the air conditioner substantially constant regardless of the speed at which the automobile is driven.
A typical automotive air conditioning system comprises a compressor having a rotor connected to the engine crankshaft by means of a drive belt and an electromagnetic clutch in such a manner that the rotor is driven by the engine when the electromagnetic clutch is engaged. The compressor compresses a refrigerant fluid and discharges the fluid from an outlet thereof into a condenser so that the fluid liquifies giving off latent heat of vaporization.
From the condenser the fluid is introduced through an expansion valve into an evaporator disposed in a passenger compartment of the automobile in which the fluid vaporizes thereby absorbing latent heat of vaporization from the air in the passenger compartment. A blower is typically provided to circulate the cooled air through the passenger compartment. From the evaporator the fluid is fed to an inlet of the compressor.
Since the compressor is driven from the automobile engine, the output of the compressor and thereby the cooling effect of the air conditioner will increase as the automobile is driven faster. It is common practice to design the compressor to provide optimum output at the most common driving speed, for example 45mph, so that at significantly higher speeds the passenger compartment will become too cold. Conversely, the air conditioner will cool the passenger compartment insufficiently when the automobile is stopped at a stoplight and the engine is idling, a condition in which the air conditioner is needed most.
Several expedients have been proposed in the prior art to prevent excessive cooling at high speed, the basis of most of them being to disengage the electromagnetic clutch and thereby disconnect the compressor from the engine when the cooling effect becomes too great. To this end, a sensor is provided which is responsive to the temperature in the passenger compartment or the engine speed to disengage the electromagnetic clutch when the sensed parameter exceeds a predetermined value. However, the pressure difference between the inlet and outlet of the compressor remains high for a substantial period of time after the compressor is stopped and if the compressor is restarted quickly it may be damaged and/or the engine may be subjected to a detrimental overload.
For this reason, a timer has been provided to prevent the compressor from being reconnected until the pressures at the inlet and outlet have had sufficient time to equalize. Another drawback of this system is that the temperature control may not be maintained with close tolerance since a substantial amount of hysterisis must be introduced into the system response to prevent rapid connection and disconnection of the compressor.
Another expedient which is known in the art is to provide an electromagnetically operated bypass valve connected between the compressor inlet and outlet. The valve is normally closed and is opened when the compressor is stopped to quickly equalize the pressure between the inlet and outlet. Although this valve improves the performance of the system, all systems of this type suffer from the drawback that they comprise a sensor, a control circuit, a valve etc. which complicate the overall configuration and increase the production cost of the air conditioning system and are subject to malfunction.